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## Low-frequency spin dynamics of the quasi-two-dimensional S = 1/2 antiferromagnet BaCdVO(PO4)2

A quasi-two-dimensional quantum magnet with competing ferro- and antiferromagnetic exchange interactions,

BaCdVO(PO4 )2, is studied using the magnetic resonance technique in a wide frequency range. The

magnetic resonance spectra of the ordered phase confirm the collinear antiferromagnetic spin structure oriented

along the a axis. The frequency-field diagram of antiferromagnetic resonance demonstrates two low-frequency

excitation branches with different energy gaps, indicating a biaxial nature of anisotropy. We observe a critical

softening of the longitudinal resonance mode in a field of 3.8 T, which is the lower boundary of a previously

proposed presaturation phase positioned below the saturation field of 6.5 T. An interpretation of the presaturation

phase as a state with variable magnetization of spin-vacancy defects is proposed. It may be considered an

alternative to the spin-nematic phase initially supposed.